The present invention relates to a switched-mode power supply.
Switched-mode power supplies usually have a transformer with a primary coil and a secondary coil, a switching element, for example a semiconductor power switch, being connected in series with the primary coil in order to apply a direct voltage in switched mode to the primary coil in accordance with a drive signal. When the switching element is closed, the primary coil consumes power that is delivered to the secondary coil when the switching element is subsequently opened. The secondary coil is a part of a secondary circuit that has output terminals for connecting a load that is supplied with an output voltage present at the output terminals. The aim of this configuration is to keep the output voltage largely constant with changing loads and for changing direct voltages present at the primary coil. For this purpose, a controlled system is provided. The controlled system includes a feedback branch and a drive circuit for providing the drive signal to the switching element. The drive circuit is supplied via the feedback branch with a signal that is dependent on the output voltage, in order to generate the drive signal in the drive circuit in dependence on the output voltage in such a manner that the output voltage is at least approximately constant.
Problems will occur if the feedback branch is broken. The signal present at the drive circuit via the feedback branch then usually corresponds to a signal that, when the feedback branch is not broken, is present when the output voltage is very small or is zero. In this case, the drive signals are generated in such a manner that a maximum of power is transmitted to the secondary side in order to bring the output voltage to a desired setpoint value. Because it is not possible to distinguish between a broken feedback branch and an output voltage of zero in the drive circuit, there is a risk that parts of the switched-mode power supply will be destroyed if the maximum of power is transmitted to the secondary side when the feedback branch is broken.
It is accordingly an object of the invention to provide a switched-mode power supply with a function for detecting a broken feedback loop that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, in which, damage to the switched-mode power supply is prevented in the event of the feedback branch being broken.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a drive circuit having a signal generating circuit for generating the drive signal and a protective circuit. The protective circuit has the effect that, when the feedback signal reaches the value of a first reference signal, no drive signal is supplied to the switching element. For this purpose, the protective circuit can drive, for example, the signal generating circuit in order to prevent the generation of a drive signal, or the protective circuit can break the connection between the signal generating circuit and the switching element, for example via a switch, if there is to be no drive to the switching element.
A broken feedback branch is usually characterized by a high signal at an input of the drive circuit to which the feedback branch is connected. In this case, the drive to the switching element is prevented by the protective circuit if the feedback signal exceeds the value of the first reference signal. In the case, where a broken feedback branch is characterized by a low signal at the input of the drive circuit, the drive to the switching element is broken if the feedback signal drops below the value of the first reference signal.
When the switched-mode power supply is switched on or at the beginning of the generation of the drive signal, respectively, the output voltage is still zero and then increases with increasing power consumption. Shortly after the switched-mode power supply is switched on, the feedback signal assumes values that correspond to the values of the feedback signal when the feedback branch is broken. To prevent the drive to the switching element to be interrupted again shortly after the switched-mode power supply is switched on, the protective circuit is constructed in such a manner that it only prevents the drive to the switching element on the feedback signal reaching the value of a first reference signal after a first period has elapsed after a start of the drive circuit.
In the switched-mode power supply according to the invention, interrupting the drive to the switching element prevents further energy from being transferred to the secondary side if, after a starting phase has elapsed, the value of the feedback signal indicates that the feedback branch is broken. This prevents the switched-mode power supply from being destroyed or damaged.
According to an embodiment of the invention, the switched-mode power supply has a first voltage supply circuit coupled to the primary coil for providing a supply voltage for the drive circuit. In this configuration, the protective circuit has the effect that no drive signal is supplied to the switching element if, during a second period after the start of the switched-mode power supply or of the drive circuit, respectively, the supply voltage reaches the value of a second reference signal and if at the same time the feedback signal has reached the value of the first reference signal.
This ensures that the drive to the switching element, and thus the power transfer to the secondary side, is interrupted if the feedback branch is broken and no load is connected to the secondary circuit. This is because, if no load is connected to the secondary circuit and power is still transferred to the secondary side because no attention is paid to a possible break in the feedback branch during the first period after the switched-mode power supply has been switched on, the voltage at the output terminals of the secondary circuit rapidly rises and could lead to damage at the switched-mode power supply. Due to the fact that the first voltage supply circuit is coupled to the primary coil, the supply voltage supplied by the first voltage supply circuit provides information about the output voltage of the secondary circuit which is also coupled to the primary coil. If the supply voltage rises very steeply within the second period after the drive circuit has been switched on and then reaches the value of the second reference signal, this indicates a non-existent secondary load and the drive to the switching element is then interrupted by the protective circuit.
According to a further embodiment of the invention, a second voltage supply circuit is provided in the drive circuit. The voltage supply circuit generates the internal supply voltages required for operating the drive circuit from the supply voltage supplied by the first voltage supply circuit. The second voltage supply circuit preferably switches off the drive circuit when the supply voltage reaches the value of a lower reference voltage and switches on the drive circuit again when the supply voltage subsequently again reaches the value of an upper reference voltage. The drive circuit is switched off by the second voltage supply circuit, for example, by interrupting the provision of the internal supply voltages.
The supply voltage drops to the value of the lower reference voltage if the drive to the switching element is interrupted and the primary coil no longer delivers any power to the first voltage supply circuit. Because the drive circuit then still consumes current from the first voltage supply circuit, the supply voltage drops. After the lower reference voltage has been reached and the drive circuit has been switched off, the first voltage supply circuit again builds up a supply voltage in order to switch the drive circuit on again via the second voltage supply circuit after an upper reference voltage has been reached. The first voltage supply circuit is preferably connected via a very large resistance to the direct voltage which is also supplied to the primary coil. A supply voltage can be built up in the first voltage supply circuit via this resistance even if the primary coil has not yet consumed any power which could be delivered to the first voltage supply circuit.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a switched-mode power supply with a function for detecting a broken feedback loop, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.